Isomerization catalyst and process for its use

ABSTRACT

The present invention is a process and a catalyst for isomerizing normal and slightly branched C 4  to C 7  hydrocarbons. The catalyst comprises a Group VIII metal on Beta zeolite.

This application is a continuation of application Ser. No. 07/150,182,filed Jan. 29, 1988, now abandoned.

FIELD OF THE INVENTION

The present invention is a process for alkane isomerization. Morespecifically, the present invention is a process for isomerizing C₄ toC₇ paraffins to increase their octane rating.

BACKGROUND OF THE INVENTION

The petroleum distillate fraction that contains C₄ to C₇ hydrocarbons isrelatively low in octane because it contains substantial amounts of lowoctane, normal paraffins. For example, normal C₅ has a blending RON of62 and normal C₆ has a blending RON of 19 (blending RON will behereinafter called "RON"). However, when these paraffins are isomerizedto form branched paraffins, their RON increases dramatically. Forexample, isopentane (2-methylbutane) has a RON of 99 and isohexane(2-methylpentane) has a RON of 83. Generally, the RON will increase witheven higher branching (e.g., 2,2-dimethylbutane has a RON of 89).

Several catalysts have been used to isomerize these lower octaneparaffins into the branched, higher octane paraffins. Examples are shownin U.S. Pat. No. 4,374,296 issue Feb. 15, 1983 to Haag et al.; U.S. Pat.No. 3,432,568 issued Mar. 11, 1969 to Miale et al.; U.S. Pat. No.3,673,267 issued June 27, 1972 to Chen et al.; and U.S. Pat. No.4,665,272 issued May 12, 1987 to Bakas et al. Haag et al. discloseisomerizing paraffins using intermediate pore zeolites, which have aconstraint index between 1 and 12. Miale et al. describehydroisomerizing saturated aliphatic and cyclic hydrocarbons bycontacting them with a dual functional catalyst comprising mordenite anda catalytic metal. Chen et al. describe a process for isomerizingparaffins using mordenite having a silica to alumina ratio between 20:1and 60:1. Bakas et al. disclose isomerizing paraffins using acrystalline aluminosilicate having a catalytic metal.

However, even though these prior art catalysts are useful, there remainsa need for a new catalyst that is: (1) highly active; (2) highlyselective for producing high octane liquid product; and (3) sulfurtolerant. That need is satisfied by the invention that is detailedbelow.

SUMMARY OF THE INVENTION

According to the present invention, a catalyst and a process for usingthe catalyst are provided for isomerizing C₄ to C₇ hydrocarbons. Thecatalyst comprises a Group VIII metal and Beta zeolite and the processcomprises contacting the catalyst with a feed having normal and branchedC₄ to C₇ hydrocarbons under isomerization conditions. Preferably, theGroup VIII metal is platinum and the feed substantially comprises normaland singly branched lower octane C₅ and C₆ hydrocarbons. To improvecatalytic activity the catalyst is preferably calcined in a steam/airmixture at an elevated temperature after impregnation with the GroupVIII metal.

Among other factors, the present invention is based on my finding that acatalyst comprising Beta zeolite and a Group VIII metal is highly activeand highly selective for isomerizing C₅ and C₆ hydrocarbons. Thecatalyst is also surprisingly selective for isomerizing C₇ hydrocarbons.Additionally, the catalyst is relatively sulfur tolerant. Furthermore,when the catalyst is calcined in a steam/air mixture, higher activityand selectivity are achieved.

More specifically, the process for alkane isomerization comprisescontacting a feed containing low octane normal and singly branched C₅and C₆ hydrocarbons and less than 0.1 ppm sulfur with a catalyst whichcomprises Beta zeolite and between 0.1% and 1.0% platinum, at atemperature between 400° F. and 600° F., at a pressure between 100 and500 psig, a H₂ /HC ratio between 1 and 8, and between 1 and 4 LHSV.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a comparative graph showing the relative conversion liquidyields in Volume %, of normal C₅, C₆ and C₇, respectively and the RON(Research Octane Number) of the isomerate produced using Beta zeoliteand mordenite containing the same amount of platinum, as given inExample 1 through 3.

FIG. 2 is a graph of relative activity of Beta zeolite and mordenitecatalyst with varying amounts of platinum impregnated therein.

DETAILED DESCRIPTION OF THE INVENTION

The following discussion presents a more detailed discussion of thepresent catalyst.

The Catalyst

Beta zeolite is a large pore zeolite. Large pore zeolites typically haveeffective pore diameters between 6 and 8 Angstroms (Å). Beta zeolite hasa pore opening of approximately 7 Å. Although its present structure isunknown, it is believed to have pore windows formed by twelve-memberedrings of silicon and aluminum atoms.

Beta zeolite is substantially shown and described in U.S. Pat. No.3,308,069 and Re 28,341 to Wadlinger et al., and Breck, "ZeoliteMolecular Sieves", p. 309 (1984), which are all hereby incorporated byreference in their entireties. These references describe a process formaking Beta zeolite, its elemental composition and formula.

Preferably, the Beta zeolite of the present invention has a silica toalumina molar ratio between 10 and 1000, more preferably between 15 and100. It is also preferable that the catalyst have high acidity. Thecatalyst's acidity can be increased by ammonium ion exchange andsubsequent calcination to yield the hydrogen form. In addition to thehydrogen form, other forms of the zeolite can be used if they aresufficiently acidic. Preferably, the catalyst is substantially free ofalkali or alkaline earth metals.

Preferably, catalysts of the present invention contain one or more GroupVIII metals, e.g., nickel, ruthenium, rhodium, palladium, iridium orplatinum. The preferred Group VIII metals are iridium, palladium, andmost preferably platinum. The preferred percentage of the Group VIIImetal in the catalyst is between 0.05% and 5%, more preferably between0.1% and 2.5%, most preferably between 0.1% and 1.0%.

Group VIII metals are preferably introduced into Beta zeolite byimpregnation, occlusion, or exchange in an aqueous solution of anappropriate salt. When it is desired to introduce two Group VIII metalsinto Beta zeolite, the operation may be carried out simultaneously orsequentially. Preferably, the Group VIII metal is highly dispersedwithin, and on, the zeolite.

By way of example, platinum can be introduced by impregnation with anaqueous solution of tetraammineplatinum (II) nitrate,tetraammineplatinum (II) hydroxide, dinitrodiamino-platinum ortetraammineplatinum (II) chloride. In an ion exchange process, platinumcan be introduced by using cationic platinum complexes such astetraammineplatinum (II) nitrate. On the other hand, when platinum isintroduced into the zeolite by occlusion a platinum complex ispreferably introduced into the zeolite during its synthesis.

After the desired metal or metals have been introduced, the catalyst ispreferably calcined in air, or air diluted with an inert gas. Morepreferably, the catalyst is calcined in a steam/air mixture at anelevated temperature after platinum impregnation.

Promoter metals can be added to the catalyst in the manner describedabove. These metals are preferably selected from Groups VIII, IVA, IB orVIB.

The zeolite is preferably bound with a porous matrix. The term "porousmatrix" includes inorganic compositions with which the zeolite willeffectively bond after appropriate thermal treatment. The matrixporosity can either be inherent or it can be introduced by chemicalmeans. Satisfactory matrices include pumie, firebrick, diatomaceousearth and inorganic oxides. Preferred inorganic oxides include alumina,silica, naturally occuring or synthetic clays (for example, bentonite,kaolin, sepiolite, attapulgite, and halloysite). Silica or alumina areespecially preferred.

Compositing the zeolite with an inorganic oxide matrix can be achievedby any suitable known method wherein the zeolite is intimately admixedwith the oxide while the latter is in a hydrous state (for example, as ahydrous salt, hydrogel, wet gelatinous precipitate, or in a dried state,or combinations thereof). A convenient method is to prepare a hydrousmono or plural oxide gel or cogel using an aqueous solution of a salt ormixture of salts (for example, sodium silicate). Ammonium hydroxide (ora similar base) is added to the solution in an amount sufficient toprecipitate the oxides in hydrous form. Then, the precipitate is washedto remove most of the water soluble salts and it is thoroughly admixedwith the zeolite which is in a finely divided state. Water orlubricating agents can be added in an amount sufficient to facilitateshaping of the mix (e.g., by extrusion).

The Process

Once the catalyst has been formed it can be employed in any of theconventional types of process equipment known to the art. It may beemployed in the form of pills, pellets, granules, broken fragments, orvarious special shapes. It can be disposed as a fixed or moving bedwithin a reaction zone, and the charging stock can be passedtherethrough as a liquid, vapor, or mixed phase, and in upward,downward, or radial flow. Alternatively, it could be prepared for use influidized-solid processes, in which the charging stock is passed upwardthrough a turbulent bed of finely divided catalyst. However, in view ofthe danger of attrition losses of the valuable catalyst and thewell-known advantages of plug flow reactors either a fixed bed system ora dense-phase moving bed system is preferred. In a fixed bed system, thefeed is preheated (by any suitable heating means) to the desiredreaction temperature and then passed into a isomerization zonecontaining the catalyst. This isomerization zone may be one or moreseparate reactors with suitable means to maintain the desiredtemperature at the entrance to each reactor. After reaction, theproducts from any of the foregoing process variants are separated fromthe catalyst, reduced to atmospheric pressure, and fractionated torecover the various components thereof. Additionally, the process can beoperated in once-through or, preferably, with recycle of normalparaffins. Recycle operations are within the knowledge of skilledpractitioners.

As discussed above, the petroleum distillate fraction preferablycontains normal and singly branched C₄ to C₇ Paraffins. The feedpreferably contains few multiply branched components such as2,2-dimethylbutane and 2,3-dimethylbutane. However, when the low octaneparaffins are isomerized to form multiply branched paraffins (such asthe dimethylbutanes, for example), the octane number increasesdramatically. This is the function of the present catalyst.

The present catalyst is highly active and highly selective forisomerizing C₄ to C₇ hydrocarbons. This high selectivity means that incomparison to prior art catalysts, such as mordenite, a higher productyield can be achieved when the two catalysts are run to a given octane.It also means that a higher octane can be achieved when the twocatalysts are run to produce a given liquid yield.

The present process comprises contacting the isomerization catalyst witha hydrocarbon feed under isomerization conditions. The feed ispreferably a light straight run fraction, boiling within the range of30° F. to 250° F. and preferably from 60° F. to 200° F. Preferably, thehydrocarbon feed for the process comprises a substantial amount of C₄ toC₇ normal and singly branched hydrocarbons, more preferably low octaneC₅ and C₆ hydrocarbons.

The present in the process is preferably between 50 psig and 1000 psig,more preferably between 100 and 500 psig. The liquid hourly spacevelocity (LHSV) is preferably between about 1 to about 10 with a valuein the range of about 1 to about 4 being more preferred. It is alsopreferable to carry out the isomerization reaction in the presence ofhydrogen. Preferably, hydrogen is added to give a hydrogen tohydrocarbon ratio (H₂ /HC) of between 0.5 and 10, more preferablybetween 1 and 8. The temperature is preferably between about 200° F. andabout 1000° F., more preferably between 400° F. and 600° F. As is wellknown to those skilled in the isomerization art, the initial selectionof the temperature within this broad range is made primarily as afunction of the desired conversion level considering the characteristicsof the feed and of the catalyst. Thereafter, to provide a relativelyconstant value for conversion, the temperature may have to be slowlyincreased during the run to compensate for any deactivation that occurs.

The present catalyst is relatively sulfur tolerant. Nonetheless, a lowsulfur feed is especially preferred in the present process. The feedpreferably contains less than 10 ppm, more preferably less than 1 ppm,and most preferably less than 0.1 ppm sulfur. In the case of a feedwhich is not already low in sulfur, acceptable levels can be reached byhydrotreating the feed in a pretreatment zone with a hydrotreatingcatalyst which is resistant to sulfur poisoning. Sulfur can besubsequently removed as hydrogen sulfide in the gas phase aftercondensation of the liquid product. An example of a suitable catalystfor this hydrodesulfurization process is an alumina-containing supportand a minor catalytic proportion of molybdenum oxide, cobalt oxideand/or nickel oxide. Hydrodesulfurization is typically conducted at 315°C. to 455° C., at 200 to 2000 psig, and at a liquid hourly spacevelocity of 1 to 5.

It is also preferable to limit the nitrogen level and the water contentof the feed. Catalysts and processes which are suitable for thesepurposes are known to those skilled in the art.

After a period of operation the catalyst can become deactivated bysulfur or coke. Sulfur and coke can be removed from the catalyst usingprocedures that are known to those skilled in the art.

The present invention will be more fully understood by reference to thefollowing examples. They are intended to be purely exemplary and are notintended to limit the scope of the invention in any way.

EXAMPLES Example 1

In a 2-liter glass liner, 8.29 gms of Mallinkrodt sodium aluminate (Na₂O.A¹ ₂ O₃. 3H₂ O) was dissolved in 259.6 gms of an aqueous solutioncontaining 20% tetraethyl ammonium hydroxide (Aldrich). 207.5 Grams ofLudox AS-30 colloidal silica was added in a thin stream while thesolution was vigorously stirred. The liner was placed in a 2-liter Parrstainless steel bomb and statically heated for 6 days at 150° C. Thesolid, precipitate product was washed and dried, after which it wasdetermined to be Beta zeolite by X-ray diffraction.

Beta zeolite was made more catalytically active by converting it intoits hydrogen (proton) form as follows. Beta zeolite was layered in athin bed and incrementally heated to 1000° F. at a rate of 150°/hr. Thetemperature was maintained at 1000° F. for 5 hours and then it wasincreased to 1100° F. for 4 more. The calcined zeolite was ion-exchangedby 4 successive treatments with N^(H) ₄ N^(O) ₃. Each treatment used thesame mass of N^(H) ₄ N^(O) ₃ to zeolite (a slurry of about 50 gr.zeolite/liter H₂ O) and the solution was refluxed for at least 2 hours.The sodium content of the zeolite was low after the fourth exchange. Thezeolite was filtered, washed, dried and recalcined as above whileomitting the 1100° F. heating step.

The final Beta zeolite as impregnated with enough tetraamine platinumnitrate to achieve 0.8 wt.% platinum. Thereafter, the impregnatedcatalyst was dried at 250° F. for approximately 14 hours then calcinedat 500° F. for 3 hours, and then reduced.

Example 2

The catalyst of Example 1 was used to isomerize a feed whichsubstantially comprised normal and branched C₅ and C₆ hydrocarbons. Theisomerization conditions where 1 LHSV, 200 psig, 6 H₂ /HC, and 520° F.The feed composition and the isomerization results are shown in Table I.

                  TABLE I                                                         ______________________________________                                        Compound (Wt. %)  Feed    Isomerate                                           ______________________________________                                        Methane           0.00    0.00                                                Ethane            0.00    0.00                                                Propane           0.00    0.71                                                Isobutane         0.04    1.84                                                N-butane          0.28    0.64                                                Iso-C.sub.5       12.03   21.45                                               N-pentane         18.93   11.37                                               2,2-DMB           0.58    9.56                                                Cyclo-C.sub.5     4.26    3.69                                                2,3-DMB           2.26    4.44                                                2MP               12.55   15.36                                               3MP               8.19    10.11                                               N-hexane          19.74   8.46                                                MCP               15.04   9.82                                                Benzene           3.75    0.00                                                CHX               1.89    2.56                                                C.sub.7 +         0.45    0.00                                                C.sub.5 + Wt. %   99.68   96.81                                               LV %              100.00  99.01                                               RON               74.50   80.08                                               ______________________________________                                    

Example 3

A commercial mordenite (Zeolon 100 H) was obtained from Norton ChemicalCompany and platinum impregnated as in Example 1. The final catalystcontained 0.8 wt.% platinum.

Example 4

This example compares the liquid yields achievable with the catalysts ofExample 1 and Example 3, which both contained 0.8% platinum.

Both catalysts were used to isomerize normal C₅, C₆, C₇ pure componentfeeds at 500° F., 100 psig, and 6H₂ /HC. The liquid yields andcorresponding octanes that were obtained for the two catalysts are shownin FIG. 1.

Example 5

Platinum was impregnated onto samples of Beta zeolite and mordenite atvarying levels. Both catalysts were used to isomerize a normal hexanefeed at 510° F., 100 psig, 3 LHSV, and 6H₂ HC. The results are shown inTable II. The relative intrinsic activity for each catalyst at lowconversion was calculated and is shown in FIG. 2.

                  TABLE II                                                        ______________________________________                                                                             Isomerate                                Catalyst Support                                                                         Pt       nC.sub.6 Conv.                                                                          LV %   RON                                      ______________________________________                                        H-Beta     2.4      84        94     76.0                                                0.8      83        97     76.1                                                 0.05    82        98     75.3                                     H-Mordenite                                                                              2.4      80        92     74.1                                                0.8      82        95     75.1                                                 0.05*   76        90     64.5                                     ______________________________________                                         *Temp. raised to 540° F. due to low activity.                     

Example 6

This example demonstrates the effect of sulfur on platinum/Beta zeoliteand platinum/mordenite catalysts. Each catalyst was contacted with thefeed of Table I at 520° F., 200 psig, 6H₂ HC, and 1 LHSV for 24 hours.Then H₂ S was continuously added to the reactor inlet H₂ and into thefeed for 24 hours. The H₂ S addition rate was sufficient to giveapproximately 250 ppm sulfur on a feed weight basis. Thereafter, the H₂S was removed and the catalysts were only contacted with clean feed andclean hydrogen. The results are shown in Table III.

                  TABLE III                                                       ______________________________________                                                          ppm Sulfur   Yield   RON                                    Catalyst Time     (Feed Wt. Basis)                                                                           (LV %)  (Calc.)                                ______________________________________                                        0.3% Pt on                                                                              0-24    0            95.3    79.9                                   Mordenite                                                                              24-48    234          86.5    80.2                                            48-72    0            95.2    79.7                                   0.3% Pt on                                                                              0-24    0            97.5    79.7                                   Beta Zeolite                                                                           24-48    234          95.3    79.7                                            48-72    0            97.3    79.2                                   ______________________________________                                    

The conventional platinum on mordenite catalyst suffers a relativelylarger loss in liquid yield than platinum on Beta zeolite catalyst.Additionally, when sulfur is withdrawn, both catalysts recover fromsulfur poisoning, but the liquid yield from the platinum/Beta zeolite isrelatively higher.

Example 7

This example demonstrates a preferred method of platinum loading whichincludes a steam calcination step after platinum impregnation.

Beta zeolite was prepared in the manner set forth in Example 1. Platinumwas impregnated onto the Beta zeolite to achieve 0.3 wt.% and thensamples of the catalyst were subjected to different calcinationtreatments. The different catalysts were then used to isomerize the feedof Table I at 200 psig, 1 LHSV, 6 H₂ /HC, for 20 hours. The results areshown in Table IV.

                                      TABLE IV                                    __________________________________________________________________________                       Isomerate Properties                                            Calcination                                                                          Reaction   2,2-DMB/                                                                            Yield                                                                              RON                                         Catalyst                                                                           Conditions                                                                           Temperature                                                                          i/nC.sub.5                                                                        nC.sub.6                                                                            (LV %)                                                                             (Calc.)                                     __________________________________________________________________________    0.3% Pt/                                                                           500° F. in                                                                    520° F.                                                                       1.7 0.6   97.4 79.5                                        Beta Air                                                                      0.3% Pt/                                                                           500° F. in                                                                    515° F.                                                                       1.8 1.0   97.8 79.9                                        Beta 50% Steam/                                                                    50% Air                                                                  0.3% Pt/                                                                           600° F. in                                                                    520° F.                                                                       2.1 0.9   97.0 80.1                                        Beta 50% Steam/                                                                    50% Air                                                                  __________________________________________________________________________

When the catalysts were calcined in a steam/air mixture, the productratios of iso to normal C₅ hydrocarbons (i/nC₅) and 2,2-dimethylbutaneto normal C₆ -hydrocarbons (2,2-DMB/nC₆) increased, which in turnincreased the product RON. Additionally, this increase was achieved at alower reaction temperature which indicated an increase in catalystactivity.

The embodiments of this invention which are exemplified above areintended solely as illustrations of the invention. They should not beinterpreted as limiting the scope of the invention to just thosefeatures which are shown or disclosed. As those familiar with this areaof research will appreciate, there are numerous variations of theinvention as defined in the following claims which may have not beenexemplified but which will achieve equivalent results.

What is claimed is:
 1. A process for isomerizing a feedstream ofpredominantly normal and singly branched C₅ to C₇ hydrocarbons to forman isomerate having a greater concentration of doubly and singlybranched hydrocarbons and a substantially higher RON than saidfeedstream comprising contacting a catalyst formed of an acidic betazeolite having between 0.05 and 5 wt% of at least one Group VIII metal,with a feedstream having predominantly normal or singly branched C₅ toC₇ components at a temperature from about 400° F. to about 600° F., at apressure between about 50 psig and about 1000 psig, an H₂ /HC ratiobetween about 0.5 and 10 and an LHSV of between about 1 and 10 to forman isomerate having a substantial increase in both the multiply andsingly branched paraffins and having an RON greater than saidfeedstream.
 2. A process in accordance with claim 1 wherein afterformation of said catalyst, including said beta Zeolite and said atleast one Group VIII metal, the catalyst is calcined in a steam and airmixture at an elevated temperature.
 3. A process in accordance withclaim 1 wherein said at least one Group VIII metal includes platinum. 4.A process in accordance with claim 3 wherein said platinum is betweenabout 0.1% wt% and about 2.5 wt % of the catalyst.
 5. A process foralkane isomerization to increase substantially the octane rating of afeedstream of predominantly normal or singly branched C₅ to C₇hydrocarbon components while maintaining a high liquid volume conversionwhich comprises contacting a catalyst comprising an acidic Beta zeoliteand between 0.1 wt % and 2.5 wt % platinum, with said feedstream havinga sulfur content of from about 0.1 ppm to about 250 ppm at a temperatureof from about 400° F. to about 600° F. at a pressure between about 100psig and 500 psig, a H₂ /HC ratio between about 1 and about 8, andbetween about 1 and about 4 LHSV to yield an isomerate of multiply andsingly branched alkanes and an octane number substantially greater thansaid feedstream and without substantial decrease in the liquid volume %of said isomerate.
 6. A process in accordance with claim 5 wherein saidplatinum is impregnated in said Beta zeolite and after formation of saidcatalyst said catalyst is calcined in a steam and air mixture at anelevated temperature and said contacting of said catalyst with saidfeedstream is at a temperature below about 550° F. to increase furtherthe yield of said multiply branched alkanes.
 7. A process in accordancewith claim 5 wherein said feedstream has a sulfur content of less thanabout 0.1 ppm.